String Equal to String Control Flow

When using = for the TDEE Calculation control flow comparison operators, the flow stops at if (activityLevel = 'sedentary'). When I use == it works as expected. I've been stuck on this for 2 hours and I've read about 3 documentations but I cant figure it out.

I know == is used for same value and meaning and === is used for same value and type, however I don't think that has anything to do with it. For a moment I thought maybe = is used for type, in this case if activityLevel equals a string, but I'm pretty sure thats not it either as I didn't come across that in documentations for =.

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Note: The commented figures in the TDEE control flow are what the results should be, not what they are.

// REE Calculation
// Males - 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) + 5 = REE
// Females - 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161 = REE

const gender = 'male'; // prompt('Are you male or female?');
const weight = 100; // prompt('How much do you weigh in KG?');
const height = 185; // prompt('What is your height in cm?');
const age = 23; // prompt('What is your age');

if (sex = 'male') {
    stepOne = 10 * weight;
    stepTwo = 6.25 * height;
    stepThree = 5 * age + 5;
    var ree = stepOne + stepTwo - stepThree;
}

if (sex = 'f') {
    stepOne = 10 * weight;
    stepTwo = 6.25 * height;
    stepThree = 5 * age - 161;
    var ree = stepOne + stepTwo - stepThree;
}

console.log(ree.toFixed(0)) // Answer is correct - 2171

// TDEE Calculation 

var activityLevel = 'moderate activity' // prompt('What is your activity level?\nsedentary/light activity/moderate activity/very active');

if (activityLevel = 'sedentary') {
    var tdee = ree * 1.2; // 2642.40
} else if (activityLevel = 'light activity') {
        var tdee = ree * 1.375; // 3027.75
    } else if (activityLevel = 'moderate activity') {
            var tdee = ree * 1.55; // 3413.10
        } else { // 3798.45
                var tdee = ree * 1.725;
            }   

console.log(tdee.toFixed(0))